Heterogeneous catalysis is a very important part of catalysis: 80% of the industrial processes use heterogeneous catalysts instead of homogeneous ones because they are much easier to handle (continuous flow products are separated from the catalyst by a simple filtration) and because they are most of the time recyclable and stable which represent major differences from homogeneous systems. However, classical heterogeneous catalysts only contain a small fraction of active sites and the leaching of metal can be observed into the liquid phase during the catalytic reaction thus leading to the pollution of the products, which are not easy to purify since the recovery of traces amounts of metal is difficult and/or costly. Concerning “supported homogeneous catalysts”, their lack of efficiency (compared to their homogeneous analogues) has prevented their use in industrial processes.
Advances in the chemistry of materials have been achieved since the discovery of nanostructured silica synthesized by Sol-Gel process via templating routes and their further functionalization by classical grafting reaction of silylated molecules. These methods have however not given access to highly active “supported homogeneous catalysts” because neither the good control of the concentration of grafted functionalities nor the homogeneous distribution of the functionalities within the materials were obtained.
Recently, new synthetic pathways to synthesize nanostructured hybrid organic-inorganic materials containing various types of organic moieties regularly distributed within the framework, either in the pore channel or in the walls, have been developed. These organic-inorganic materials were prepared by sol-gel process by hydrolysis and condensation of organosilylated precursors with or without dilutants in presence of surfactants as structure directing agents. The first example of such materials containing organic moieties in the channel pores was described in 1996 by Mann et al. (Burkett et al., Chem. Commun. 1996, 1367) and Macquarrie D. J. et al. {Macquarrie D. J. et al., Chem. Comm 1996, 1961), whereas the first example of a nanostructured material functionalized in the walls was only achieved in 1999 by Inagaki S (Inagaki et al., J. Am. Chem. Soc. 1999, vol 121, 9611). In this type of synthesis, the structuration of the material as well as the regular distribution of the organic moieties within the framework are attributed to interactions between the structure-directing agent (surfactant) and the silylated precursors (organoalkoxysilane or alcoxysilane). Numerous advanced hybrid nanostructured materials designed for various applications (optic, gas separation (Corriu et al., Patent WO 2002/076991), ions separation (B. Lee et al. Langmuir, 2005, vol. 21, 5372) have been developed so far. Recently, Corriu R. J. P. et al. have investigated the structuration of such materials and have demonstrated the regular distribution of the organic moieties within the materials wherever their localisation (in the pores or in the walls) in the J. Mater. Chem., 2005, 15, 4285-429, in the New J. Chem., 2006, 30, 1077-1082., and in the J. Mater. Chem., 2005, 15, 803-809. Until now, the use of these types of materials in catalytic applications has been limited only to a few examples such as acido-basic catalysis or hydrogenation/oxidation reactions, in fact only two examples of these above described materials have been currently used as catalysts for challenging reactions such as metathesis of olefins (X Elias et al., Adv. Synth. Catal., 2007, Vol 349, 1701) and C—C coupling (V. Polshettiwar et al., Tetrahedron Letters, 2007, Vol 48, 5363) because most of the time they are limited to simple systems, typically not sensitive towards H2O, oxygen.
A large number of reactions uses homogeneous catalysts based on organometallic complex which are sensitive to protic media and oxygen. As an example, of these homogeneous catalysts, the family of the metallo-monocarbenes as published by Grubbs and Hermann are part of the most highly active catalysts ever described, but these chemical entities are very sensitive and deactivate rapidly during the catalytic process. Moreover, these catalysts are not suitable for the synthesis of all the chemical products, like pharmaceutical active principles, because they are difficult to separate from the reaction products and are not quantitatively recovered at the end of the reaction.
Surprisingly hybrid organic-inorganic materials that contain stabilized carbene could be prepared with a good stability of the metallo-carbene species and more surprisingly, their catalytic performances are at least comparable or greater to these of the corresponding homogeneous catalysts.
Moreover, the synthesis of these materials could be achieved by Sol-Gel process using surfactants to regularly distribute the metallo-carbene moieties within the material. Noteworthy, after the catalytic process, the catalytic material can be completely recovered, recycled, and no leaching of metal was observed in the reaction mixture.